So what exactly does this color mean?These kind of basses don't have any finish (poly/nitro/etc.) ? and does this mean that repainting such a bass doesn't require any sanding/heat machine/etc ?
Sounds to me like it just means that an open grained wood (like mahogany or walnut) was used and was not grain filled before finish. Nothing special, and yes, would still require sanding before refinishing as there is still finish on the wood.